Communications channel testing arrangement

ABSTRACT

A switching arrangement that can be connected in a communications channel that comprises a pair of lines between sets of terminal equipment, which comprises: (i) a series switch (1,2) for connection in each of the lines; (ii) a shunt switch for connection between the lines; and (iii) a control circuit (5) that can actuate the series switches (1,2) and can actuate the shunt switch on receipt of a signal sent along the channel: wherein the control circuit (5) can actuate the shunt switch and the series switches (1,2) on receipt of one or more signals but the shunt switch will remain closed over a different time period than that during which the series switches remain open, in order to allow different tests to be performed on the channel, and wherein one or more of the switches comprises a solid state switch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to communications or other circuits, andespecially to maintenance termination units for use intelecommunications circuits such as telephone circuits.

2. Introduction of the Invention

In recent years, and especially in view of deregulation of manytelephone systems, privately owned communication equipment hasincreasingly been installed in the premises of subscribers to thesystem, with the result that it is often necessary to determine whetherany fault is located in the telephone line, or in the subscriber'spremises, i.e. in the subscriber's equipment or cabling, in order todetermine whose responsibility it is to repair the fault. It is highlyadvantageous economically if this determination can be performedremotely by sending an appropriate signal from the local exchange alongthe line, thereby obviating the necessity to send any telephone companypersonnel to the subscriber's premises.

In order to test the telephone line for any faults it is necessaryfirstly to install a so-called "maintenance termination unit" or MTU inthe line at the subscriber's premises which can disconnect thesubscriber equipment from the line (often called sectionalizing theline) and connect the a and b or tip and ring lines on receipt of theappropriate signals from the exchange. During the line testing proceduredeterminations will typically be made of the line to line resistance andof the first and second line to ground resistance. Also the linecontinuity can be determined by detecting the presence of the MTUelectronically.

Various forms of MTU are described in the prior art, and may employsolid state (such as silicon) switches or electrical relays. Solid stateswitching devices are preferably used in the present invention in viewof their greater reliability and lower costs as compared witharrangements that incorporate relays. A number of such devices aredescribed, for example, in U.S. Pat. No. 4,710,949 to Om Ahuja. Thisdevice comprises a pair of voltage sensitive switches, one located ineach of the tip and ring lines, and a distinctive termination connectingthe tip and ring lines on the subscriber side of the voltage-sensitiveswitches. The voltage-sensistive switches may each have a thresholdvoltage of about 16 volts so that they are closed in normal operation bythe 48 volt battery voltage but will open when this is replaced by atest voltage below about 32 volts in order to test the line-to-groundand tip-to-ring impedances. The distinctive termination may, forexample, comprise a back-to-back diode and Zener diode which willexhibit an asymmetric resistance when large voltages (higher than theoperating voltages) of different polarity are applied.

Although this form of MTU will perform adequately to sectionalize afault in a line, it suffers from the problem that it requires theprovision of ringing by-pass capacitors in the signal path in parallelwith the voltage-sensitive switches. These capacitors are necessarybecause the amplitude of the ringing signal (about 80V RMS) which issuperimposed on the 48V DC battery voltage, is sufficiently large forthe polarity of the resultant signal to change during the ringing signalcycles and to cause unacceptably large crossover distortion due toopening of the voltage-sensistive switches, at the crossover points ofthe ringing signal. Because the ringing frequency is relatively low,about 20 Hz, a large capacitance is required for the ringing by-passcapacitors, typically in the order of 1.0 μF. Because the capacitors areconnected in the signal line they need to have a high voltage rating inorder to withstand normal electrical transients, etc; which increasestheir cost and physical size. In addition they can provide a lowimpedance path for transients.

Also, to use the voltage-sensitive switches of the prior art linemeasurements are in general carried out at about 10V. Unfortunately, lowvoltage measurements suffer from inaccuracies due to electrical noisepicked up from such sources as adjacent power or communication lines orfrom corroded terminals that produce a battery-like e.m.f. in additionto thermal noise.

This problem is overcome according to our copending British patentapplication No. 9213980.7 which claims a switching arrangement that canbe connected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, for example between a subscriber andan exchange, which comprises:

(i) a DC voltage window detector circuit (referred to herein as a windowdetector circuit or simply as a window circuit) that is connectedbetween the lines and is responsive to the voltage between the lines;and

(ii) one or more switching circuits connected in or between the linesthat can be actuated by the window detector circuit when, and only when,the voltage between the lines is within a predetermined band.

so that the or each switching circuit can be remotely actuated by meansof a DC signal on the line, the arrangement including a low pass filterassociated with the or each switching circuit having a cut off frequencythat is sufficiently low to prevent the switching circuit(s) beingactuated by a ringing signal on the channel.

In addition, our copending British patent application No. 9213992.2claims:

A switching arrangement that can be connected in a communicationschannel comprising a pair of lines, between sets of terminal equipment,which comprises:

(i) a pair of series switching circuits, each of which can be seriesconnected in one of the lines and will open when subjected to anovercurrent in its associated line; and/or

(ii) one or more shunt switching circuits connected between the lines,or between one or both the lines and earth, and will close whensubjected to an overvoltage in its associated line;

wherein the or each switching circuit can be remotely actuated by meansof a test signal sent along the channel.

GB 2030820 (Fields) discloses a remotely-actuated line testing block.When a test voltage of a first polarity from a central office is appliedacross one of the tip and ring wires of a telephone circuit a capacitoris charged. Upon release of the test voltage the capacitor activates afirst switch which connects the tip and ring together. The capacitoractivates a second switch which disconnects the subscriber's equipment.The first switch releases after about half of a test interval with thesecond switch releasing at the end thereof to restore normal lineconnections. The switches are mechanical relays.

SUMMARY OF THE INVENTION

We have now devised various improvements to such switching arrangements.

Thus, the invention provides a switching arrangement that can beconnected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines;

(ii) a shunt switch for connection between the lines; and

(iii) a control circuit that can actuate the series switches and canactuate the shunt switch on receipt of a signal sent along the channel;

wherein the control circuit can actuate the shunt switch and the seriesswitches on receipt of one or more signals but the shunt switch willremain closed over a different time period than that during which theseries switches remain open, in order to allow different tests to beperformed on the channel, and wherein one or more of the switchescomprises a solid state switch.

The invention also provides a switching arrangement that can beconnected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines and which willswitch to an open state when subjected to an overcurrent;

(ii) a shunt switch for connection between the lines; and

(iii) a control circuit that can actuate the series switches and canactuate the shunt switch on receipt of a signal sent along the channel;

wherein the control circuit can actuate the shunt switch and the seriesswitches on receipt of one or more signals, but the shunt switch willremain closed over a different time period than that during which theseries switches remain open, in order to allow different tests to beperformed on the channel.

The invention further provides a switching arrangement that can beconnected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines;

(ii) a shunt switch for connection between the lines; and

(iii) a control circuit connected between the lines of thecommunications channel and that can actuate the series switches and canactuate the shunt switch on receipt of a signal sent along the channel;

wherein the control circuit can actuate the shunt switch and the seriesswitches on receipt of one or more signals, but the shunt switch willremain closed over a different time period than that during which theseries switches remain open, in order to allow different tests to beperformed on the channel.

In another embodiment the invention provides a switching arrangementthat can be connected in a communications channel that comprises a pairof lines between sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines;

(ii) a shunt switch for connection between the lines; and

(iii) a control circuit comprising a DC voltage window detector circuitthat is connected between the lines and is responsive to voltage betweenthe lines and that can actuate the series switches and can actuate theshunt switch on receipt of a signal sent along the channel;

wherein the control circuit can actuate the shunt switch and the seriesswitches on receipt of one or more signals, but the shunt switch willremain closed over a different time period than that during which theseries switches remain open, in order to allow different tests to beperformed on the channel.

The invention also provides a switching arrangement that can beconnected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines;

(ii) a shunt switch for connection between the lines; and

(iii) a control circuit that can actuate the series switches and canactuate the shunt switch on receipt of a signal sent along the channel;

wherein the control circuit can actuate the shunt switch and the seriesswitches on receipt of one or more signals, but the shunt switch willremain closed over a different time period than that during which theseries switches remain open, in order to allow different tests to beperformed on the channel; and either

(a) the shunt switch is intended to be located on the exchange side ofthe series switches, and, after the switches have been actuated by thecontrol circuit, the shunt switch will open before the series switchesclose; or

(b) the shunt switch is intended to be located on the subscriber side ofthe series switch, and after the switches have been actuated by thecontrol circuit, the series switches will close before the shuntswitches open.

The invention yet further provides a switching arrangement that can beconnected in a communications channel that comprises a pair of linesbetween sets of terminal equipment, which comprises:

(i) a series switch for connection in each of the lines;

(ii) a voltage generator that normally, biases the switches closed, thevoltage generator taking power from the voltage between the lines; and

(iii) a control circuit that controls the current input to the voltagegenerator,

the control circuit being capable of opening the series switches byinterrupting current to the voltage generator in response to a signalsent along the communications channel.

DETAILED DESCRIPTION OF THE INVENTION

In the invention described in British application No. 9213980.7, seriesswitches in the lines are opened for subscriber disconnect tests (e.g.line-to-line and line-to-ground resistance) and the shunt switch isclosed for a loop back test, on receipt of different signals from theexchange, for example on receive of DC signals of different polarity.This, however, can cause a problem during testing if the tip and ring (aand b) line have inadvertently been reversed when installed, and wherethat problem is likely, we prefer that both the shunt and seriesswitches are activated on receipt of a single signal.

Even where separate signals are used for the two tests, the differentdurations of the series and shunt switch activations can be desirable.This is because the two types of testing will in general take differentlengths of time, the loop-back test being quicker than the disconnecttest.

The invention then has the advantage that, since both types of test canbe performed after receipt of a single signal, e.g. a DC voltage appliedbetween the lines, it is possible to perform the tests without thepolarity of the signal being important.

The arrangement may be formed in a number of configurations. For examplethe shunt switch may be intended to be located on the exchange side ofthe series switches, in which case, after the switches have beenactuated by the control circuit (to close the shunt switch and open theseries switches), the shunt switch will open before the series switchesclose. This allows a loop back test to be carried out first and then aninsulation test to be carried out. Alternatively, the shunt switch maybe intended to be located on the subscriber side of the series switches,in which case, after the switches have been actuated by the controlcircuit, the series switches will close before the shunt switch opens.This allows an insulation test to be carried out, followed by aloop-back test.

In the broadest aspect of the invention the arrangement may be operatedby a range of signals. e.g. AC signals of a predetermined frequency orby a DC signal. Preferably the arrangement responds to a DC signal as inthe case of our copending British applications mentioned above, in whichcase the control circuit preferably includes a window detector circuit.

Normally the window detector circuit will allow current to flow throughit only when the line voltage is within a predetermined band (which willbe above normal signaling voltages), i.e. the current that flows throughthe window circuit is significantly greater when the line voltage iswithin the band than when it is outside the band, although at very highapplied line voltages the leakage current through the window circuit mayapproach or even exceed the within-band current flow.

The window detector circuit that determines the applied voltage at whichthe switches will open and close may include a Zener diode that sets thelower limit of the applied voltage that will cause current to flow. Theupper limit of the applied voltage may conveniently be set by means ofan overcurrent switching circuit that will open when the current passingthrough the circuit, and hence the voltage applied across it, exceeds apredetermined value, or it may include a further zener diode.

Preferably the control circuit is located on the subscriber side of theseries switches. Such an arrangement has the advantage that, when theswitches are opened for a subscriber disconnect test, the leakagecurrent can be reduced to a very low value. e.g. below 50 μA andespecially below 10 μA. This may be achieved by including a pair ofcapacitors in the control circuit which are charged when the appropriatesignal is received from the exchange, and will then be discharged inorder to actuate the switches (this discharge preferably using circuitshaving different time constants in order to actuate the shunt and seriesswitches for different lengths of time).

In addition to the ability of the arrangement to be operated in order tolocate a fault in the channel, it is possible according to a preferredaspect of the invention, for the circuits to be capable of protectingthe system from overcurrents and overvoltages by opening and closingrespectively. This can be achieved by employing the switches describedbelow.

The series switch in each of the lines is a solid state switch, normallyformed in silicon. In one form of arrangement, the switches may comprisea switching transistor whose input voltage is controlled by anovercurrent control element which switches on when the switching circuitis subjected to an overcurrent, thereby turning the switching transistoroff. Such a circuit on its own will only switch in response to anovercurrent in its associated line. However, the circuit may include atest control element that also controls the input voltage of theswitching transistor. The test control element turns, on when currentflows in the window circuit, thereby turning the switching transistoroff. Thus, in this way the series switching circuits can be actuatedeither remotely or by an overcurrent in the line. The control elementsmay be formed from any of a number of devices, and the choice of controlelement will depend to some extent on the type of switching transistoremployed. The overcurrent control element may, for example, comprise atransistor whose base or gate is held in a potential divider that spansthe switching transistor so that the base-emitter or gate-source voltageincreases as the current in the line increases. Alternatively thecontrol element may comprise a comparator that compares a fraction ofthe voltage across the switching transistor with a reference voltage andopens the switch if the fraction is greater than the reference voltage,as described in our copending international application No.PCT/GB91/02215. If a normally-on FET such as a JFET or a depletion modeMOSFET is employed as the switching transistor, a negative voltagegenerator, e.g. a charge pump, or an optocoupler may be employed as thecontrol element, as described in our copending British application No.9114717.3 The disclosures of these specifications are incorporatedherein by reference.

A particularly preferred form of series switch that can be used in thepresent invention is described in our British patent application No.9223773, entitled "Switching Arrangement", the disclosure of which isincorporated herein by reference. That arrangement comprises anarrangement for connection in an electrical circuit, which comprises:

(1) a pair of FETs that are series connected in a line of the circuitwith their sources connected together or with their drains connectedtogether and whose states can be altered by means of a voltage acting ontheir gates; and

(2) a control connected to the gate of at least one of the FETs;

the control being responsive to an overcurrent on the line therebyaltering the states of at least one of the FETs.

Preferably it comprises a pair of n-channel enhancement mode MOSFETswhose gates are connected to a voltage generator. When a voltage isapplied to the gates the switch will be closed (current flowing throughthe reverse biased FET due to its parasitic diode), and when the voltageis removed the switch will be open. A pair of control transistors arepreferably connected between the source and gate of the MOSFETs and arepreferably controlled by the voltage appearing across the arrangement sothat they will cause the arrangement to open when an overcurrent isexperienced. We prefer that the control resistors or other controlcauses V_(GS) of at least one n-channel FET to decrease as V_(DS) ofthat FET increases, thereby causing that FET to exhibit foldbackbehavior. Alternatively the control can cause V_(GS) of at least onep-channel FET to increase as V_(DS) of that FET increases therebycausing that FET to exhibit foldback behavior. The foldback process cantherefore provide positive feedback.

Preferably all components of the arrangement take their power from thecurrent in the lines or from the voltage drop between them so that noseparate power supply rails are needed.

It is quite possible to produce a number of arrangements each having adifferent AC actuation voltage window so that they can be connected atvarious points along a long channel in order to divide the channel intosections for locating a fault.

BRIEF DESCRIPTION OF THE DRAWINGS

One form of arrangement according to the present invention will now bedescribed by way of example with reference to the accompanying drawingsin which:

FIG. 1 is a block diagram of an arrangement according to the invention;

FIG. 2 is a circuit diagram of an arrangement as shown in FIG. 1; and

FIG. 3 is a circuit diagram of an alternative arrangement as shown inFIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the accompanying drawings, FIG. 1 is a block diagram of anMTU circuit according to the invention. It consists of two seriesswitches 1 and 2 connected in the line to provide a subscriberdisconnect function as well as a protection function. These switches arebiased into conduction by a voltage generator 3 which is powered fromthe line, on the exchange side of the MTU. The series switches arebidirectional and can therefore work for any polarity of voltage thatmay appear on the line. The voltage generator which provides the biasingvoltage for the switches is regulated by a current regulator circuit 4which limits a current that can be supplied to the voltage generator 3.

The MTU function of the circuit is activated by a control circuit 5which interprets a valid activation voltage that appears on the line andif it is the correct voltage it sends a signal to open the seriesswitches 1 and 2 and to place a loopback resistance in the line forloopback measurement. The loopback resistance enables the resistance ofthe line to be measured. The loopback function is carried out by turningon a transistor switch that is series connected to the loopback resistoracross the line, via the bridge rectifier 6. The subscriber disconnectfunction is carried out by turning on a transistor in order to disablethe current regulating circuit 4 which then removes the supply currentfrom the voltage generator 3. This action causes the output of thevoltage generator 3 to fall to zero, removing bias from the seriesswitches 1 and 2, and thus opening them.

The control circuit 5 sends out the two signals for a different periodof time. One signal is for the subscriber disconnect function and theother is for the loopback function. Both signals are active for Xseconds (preferably 2 to 10 especially about 5) so that the loopresistance can be measured with the subscriber disconnected. After Xseconds the the loop resistor is disconnected from the line and a lineinsulation resistance measurement can be made. After a total of Yseconds (preferably 10-30, especially about 15), where X is less than Y(preferably from one half to one quarter, especially about one third),the control signal is reduced to zero and the voltage generator 3 startsto operate, turning on the series switches 1 and 2, and reconnecting thesubscriber to the exchange.

The control circuit 5 sends out a control signal when a valid DCvoltage, of either polarity, is on the line. In order to operate the MTUthe activation voltage must be between an upper and a lower voltagelimit for the control circuit to operate. This is defined as the windowvoltage. The circuit has a low pass filter built into it to ignore theAC ringing voltage on the line.

When a valid activation voltage, within the window, is present on theline a timing circuit charges up so that the switches may be activatedfor X and Y seconds. The switches are not actually actuated until thevoltage across the lines is out of the upper and lower limit of thewindow. This is to allow the capacitors in the timing circuits to chargeup. Once the voltage has been increased or decreased out of the windowthe test functions will actuate.

Overcurrent protection for the subscriber (or for the exchange) may beincorporated into the series switches if required. Standard overvoltageprotection components may be placed before the MTU to protect the MTU(and the subscribers) from any overvoltage spikes that may appear on theline.

FIG. 2 is one possible circuit diagram of the arrangement shown in FIG.1.

The series switching element in the tip line is described below. The onein the other line is identical in operation.

The two series switches 1 and 2 may be identical, the switching circuit1 comprising for example two n-channel field effect transistors Q1 andQ2 connected with their sources together such that one transistor isalways forward biased and the other reversed biased (although which iswhich will depend on the polarity of the voltage on the line). Inreverse bias the line current will flow through the "parasitic" drainsource diode of the FET, giving a very low voltage drop. This allows thecircuit to exhibit a linear AC characteristic. The forward biasedtransistor has its gate biased on by the voltage generator circuit. Thevoltage generator circuit produces a voltage that is higher than thesource voltage plus the gate threshold of the FET in order to bias theFET on. The purpose of resistor R1 is to tie the gate of the FET to thesource so that the gate terminal does not float.

Two bipolar overcurrent control transistors Q3 and Q4 have theircollectors and emitters connected to the gates and sources of the FETswitching transistors. When an overcurrent occurs on the line, a voltagewill develop across the main switching transistor. This voltage isconnected to the base of the overcurrent control transistors by thevoltage dividers R3 and R2 spanning Q3 and R4 and R5 spanning Q4. Whenthe voltage at the base of the overcurrent control transistor rises to0.7 of a volt, the transistor will turn on and will short the gate ofthe switching transistor to its source, thus turning the switchingtransistor off. Capacitors Cl and C2 are provided to prevent the circuitfrom switching off when power is initially applied to the line. Theyalso prevent spurious current spikes that normally appear on the linefrom activating the overcurrent control transistors.

Once the FET has been "tripped" into its high resistance state, all ofthe voltage available appears across it. This has the effect of latchingthe overcurrent protection into the high resistance state, until eitherthe supply is removed, or the load is removed momentarily from thecircuit (such as the phone being placed onhook).

The voltage generator circuit 3 consists of two opto-isolators 10 and11. These have an LED input which is powered from the line, via bridgerectifier BR1, and the current regulating circuit. The LED will emitlight onto the photovoltaic diodes and a voltage will be generated atthe terminals of these diodes. This voltage is then fed to the gate andsource terminals of switching transistors Q1 and Q2 in the case ofdiodes D1 and D6, and to the source and gate terminals of switchingtransistors Q13 and Q14 in the case of diodes D2 and D3. This has theeffect of biasing the forward biased switching transistors intoconduction.

The current regulating circuit 4 regulates the supply current to thevoltage generator circuit. The current regulating circuit consists oftransistors Q5 and Q6, and resistors R7 and R6. Resistor R7 providesgate bias to transistor Q5, so that Q5 will conduct. Resistor R6 is acurrent sense resistor such that when the maximum supply currentrequired by the voltage generator circuit is reached, a voltage equal tothe gate voltage of transistor Q6 will be developed across it. This hasthe effect of turning transistor Q6 partly ON which removes voltage fromthe gate of transistor Q5. The drain source resistance of Q5 willincrease because of this and the current will be limited to the valuerequired by the voltage generator circuit.

A window detector circuit is connected between the lines, on thesubscriber side of the MTU and is part of the control circuit. Itcomprises transistors Q11 and Q12, zener diodes D10 and D12, resistorsR15, R17, R18 and R19, and some steering diodes to connect the circuitto the line. When a voltage of, say, between 75V and 90V appears on theline, which is the test activation voltage, base current will flow outof transistor Q12 and through zener diode D10, thus turning Q12 on. Ifthe voltage is above 90V then base current will flow out of transistorQ11 and through zener diode D12 thus turning Q11 on. This has the effectof removing base drive from transistor Q12, turning it off. This meansthat transistor Q12 conducts for a voltage between the lines of 75V to90V (the window voltage).

When transistor Q12 conducts, a current will flow through resistors R15and R19. The current flowing through these resistors will cause avoltage to be developed across resistor R15 and this voltage is passedonto the timed test control circuits 12 and 13 via a low pass filterformed by capacitor C5, resistor R14 and diode D9 in parallel with R14.When the unit is subjected to a ringing signal a train of unipolarpulses of short duration is developed across resistor R15. The purposeof diode D9 is to enable capacitor C5 to discharge faster than it ischarged when the low pass filter is subjected to this train of pulsesand so limits the voltage developed across capacitor C5 to about 0.7 ofa volt.

The filtering circuit is connected to a timing element, formed bycapacitor C4 and resistor R11 in the remote disconnect test controlcircuit 12, by diode D8. Diode D8 allows the timing element to chargequickly, but prevents it discharging through the same path that it wascharged from. The timing element is connected to the test controltransistor Q8 via resistor R13.

When a valid activation voltage of between 75V and 90V is applied to theline the timing capacitors will start to charge. At the same timecurrent will flow through resistor R9, which is connected to the windowcircuit resistor R15, such that base current will flow into the base oftransistor Q9, turning it on. This has the function of shorting the gateof the test control transistor Q8 to its source, preventing it fromcarrying out the test function. As soon as the valid activation voltageis removed from the line transistor Q9 will cease to conduct, and thevoltage at the timing element formed by capacitor C4 and resistor R11will cause transistor Q8 to turn on. Q8 remains on for as long ascapacitor C4 remains charged above the gate voltage of Q8. The purposeof having transistor Q9 is to allow the timing capacitor to chargebefore opening the series switches.

When transistor Q8 is turned on it has the effect of removing gate drivefrom the current regulating transistor Q5, turning Q5 off. This stopsthe current flowing through the voltage generator circuit, removing biasfrom the series switches, thus opening the series switches, for a periodof time defined by the timing circuit.

The other timed test control circuit 13, for the loopback element,operates in a similar fashion, but the capacitor size of the timingelement is half that of the remote disconnect timed test controlcircuit. This means that the loopback function will be activated forhalf of the time of the disconnect function. When a valid activationvoltage is present on the line capacitor C3 will charge. When theactivation voltage is removed transistor Q10 will turn off in a similarfashion to Q9 in the other circuit, and the voltage at the timingelement formed by capacitor C3 and resistor RIO will be connected to thegate of test control transistor Q7, via resistor R12. Test controltransistor Q7 will turn on, connecting the loopback resistance R8 intothe line, providing the loopback function.

In this circuit the duration of the loopback function is half that ofthe subscriber disconnect function. This enables the line resistance tobe measured with the subscriber disconnected, and then the insulationresistance of the line to be measured, without anything else connectedacross the lines.

The circuit shown in FIG. 3 is similar to that shown in FIG. 2 exceptthat charge pumps are used as the voltage generators instead ofoptoisolators. An advantage of such charge pumps is a reduced supplycurrent, of say, 30 microamps. Also a reduced voltage drop across theseries switches can be achieved if the overcurrent protection circuitryis omitted.

We claim:
 1. A switching arrangement which can be connected in acommunications channel, the communications channel comprising a pair oflines, the switching arrangement, which, in use, is connected betweensets of terminal equipment, and which comprises:(i) first and secondseries switches, each of which, in use, is series connected in one ofthe lines; (ii) a shunt switch which, in use, is connected between thelines; and (iii) a control circuit which can actuate the series switchesand can actuate the shunt switch on receipt of a signal sent along thechannel;wherein (a) the control circuit can actuate the shunt switch andthe series switches on receipt of one or more signals, (b) the shuntswitch will remain closed over a different time period than that duringwhich the series switches remain open, in order to allow different teststo be performed on the channel, and (c) at least one of the followingconditions is present:(1) one or more of the switches comprises a solidstate switch, (2) the series switch will switch to an open state whensubjected to an overcurrent, (3) the control circuit, in use, isconnected between the lines of the communications channel, and (4) thecontrol circuit comprises a DC voltage window detector circuit that isconnected between the lines and is responsive to voltage between thelines.
 2. An arrangement according to claim 1, in which the solid stateswitch comprises a switching transistor.
 3. An arrangement according toclaim 1, in which at least one of the series switches comprises a pairof FETs, each FET comprising a source, a drain, and a gate.
 4. Anarrangement according to claim 3, in which the FETs(a) compriseenhancement mode FETs with their sources connected together, and (b) canbe biased into conduction by means of a voltage source acting on theirgates.
 5. An arrangement according to claim 3, in which at least one ofthe series switches additionally comprises a pair of controltransistors, each control transistor being connected between the gateand source of one of the FETs.
 6. An arrangement according to claim 5,in which each control transistor is held in a voltage divider that spansthe series switch.
 7. A switching arrangement which can be connected ina communications channel, the communications channel comprising a pairof lines, the switching arrangement which in use is connected betweensets of terminal equipment, and which comprises:(i) first and secondseries switches, each of which, in use, is series connected in one ofthe lines, said series switch having an exchange side and a subscriberside; (ii) a shunt switch which, in use, is connected between the lines;and (iii) a control circuit which can actuate the series switches andcan actuate the shunt switch on receipt of a signal sent along thechannel;wherein (1) the control circuit can actuate the shunt switch andthe series switches on receipt of one or more signals, (2) the shuntswitch will remain closed over a different time period than that duringwhich the series switches remain open, in order to allow different teststo be performed on the channel, and (3) either(a) the shunt switch inuse is located on the exchange side of the series switches, and, afterthe shunt switch and series switches have been actuated by the controlcircuit, the shunt switch will open before the series switches close; or(b) the shunt switch in use is located on the subscriber side of theseries switch, and after the shunt switch and series switches have beenactuated by the control circuit, the series switches will close beforethe shunt switch opens.
 8. An arrangement according to claim 7, in whichthe control circuit bridges the pair of lines on the subscriber side ofthe shunt switch.
 9. An arrangement according to claim 7, in which thecontrol circuit bridges the pair of lines on the subscriber side of theshunt switch and the series switches.
 10. An arrangement according toclaim 1, in which both the shunt switch and the series switches areactivated on receipt of a single signal.
 11. An arrangement according toclaim 1 wherein the series switch has an exchange side and a subscriberside, the shunt switch, in use, is located on the exchange side of theseries switch, and, after the switches have been actuated by the controlcircuit, the shunt switch will open before the series switches close.12. An arrangement according to claim 1, wherein the series switch hasan exchange side and a subscriber side, the shunt switch, in use, islocated on the subscriber side of the series switch, and, after theswitches have been actuated by the control circuit, the series switcheswill close before the shunt switches open.
 13. An arrangement accordingto claim 1, wherein the series switches are controlled by a voltagegenerator that takes its power from voltage appearing between the lines,the voltage generator being controlled by the control circuit.
 14. Aswitching arrangement which can be connected in a communicationschannel, the communications channel comprising a pair of lines, theswitching arrangement, which, in use, is connected between sets ofterminal equipment, and which comprises:(i) first and second seriesswitches, each of which, in use, is series connected in one of thelines; (ii) a voltage generator that normally biases the series switchesclosed, the voltage generator taking power from the voltage between thelines; (iii) a control circuit that controls the current input to thevoltage generator, the control circuit being capable of opening theseries switches by interrupting current to the voltage generator inresponse to first signal sent along the communications channel; and (iv)one or more shunt switches connected between the lines, which shuntswitch can activate in response to second signal sent along thecommunications channel.
 15. An arrangement according to claim 7, inwhich both the shunt switch and the series switches are activated onreceipt of a single signal.
 16. An arrangement according to claim 7wherein the series switch has an exchange side and a subscriber side,the shunt switch, in use, is located on the exchange side of the seriesswitch, and, after the switches have been actuated by the controlcircuit, the shunt switch will open before the series switches close.17. An arrangement according to claim 7 wherein the series switches eachhave an exchange side and a subscriber side, the shunt switch, in use,is located on the subscriber side of the series switches, and, after theshunt switch and series switches have been actuated by the controlcircuit, the series switches will close before the shunt switches open.18. An arrangement according to claim 7, wherein the series switches arecontrolled by a voltage generator that takes its power from voltageappearing between the lines, the voltage generator being controlled bythe control circuit.